1. Field of the Invention
The present invention relates to a pneumatic tire in which a rib sectionalized by main grooves extending along a tire circumferential direction is provided in a tread surface.
2. Description of the Related Art
In a tire for a passenger car in recent years, attaching importance to a steering stability at a time of going straight at a high speed, a pattern having a rib sectionalized by main grooves extending along a tire circumferential direction in a center portion of a tread surface is a mainstream. In the tire mentioned above, particularly in the case that the tire is installed to a rear side, there is a tendency that an irregular wear referred to as a so-called center wear that the rib provided in the center portion of the tread surface wears in an early stage is generated. This tendency is as remarkable as an aspect ratio of the tire is lower.
Japanese Unexamined Patent Publication No. 2000-16026 describes a tire structured such that an irregular wear can be suppressed by providing a small circular hole in the vicinity of an end in a width direction of a land portion. However, this technique suppresses a ground pressure increase in the vicinity of the end in the width direction in a lateral force input side, for suppressing an irregular wear (particularly, taper wear in a rib) under a condition that the input in the tire width direction is dominating, in a heavy load tire. On the other hand, the center wear mentioned above is an irregular wear under a condition that a compression of the rib by an input in a tire diametrical direction is dominating, and cannot be sufficiently suppressed even by providing the small hole in the end in the width direction of the rib. Japanese Unexamined Patent Publication No. 2000-16026 positions that an intended effect cannot be obtained in the case that the small hole is provided in a center side of the land portion.
Japanese Unexamined Patent Publication No. 8-85308 describes a pneumatic tire in which a lot of circular or oval dimples are arranged in a rib, and a peripheral length in a foot print at a time of applying a load is made substantially identical between a shoulder rib and an inside rib. However, this technique only adjusts the circumferential length of each of the ribs by collapsing the dimples from a tire circumferential direction, and is not enough to suppress the center wear as mentioned above.